This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Enveloped viruses use specialized protein machinery to fuse the viral membrane with that of the host cell in a programmed fashion. Influenza A virus is a paradigm for understanding viral entry of host cells via receptor-mediated endocytosis. The hemagglutinin (HA) envelope glycoprotein mediates influenza virus membrane fusion. While certain facets of HA-mediated fusion have been characterized, significant gaps remain in the structural characterization of the molecular conformations that actually drive membrane fusion, in our understanding of how HA switches conformations, and in our understanding of the interplay between HA conformational change and membrane deformation.